1. Field of the Invention
This invention relates to a circuit breaker with an adjustable magnetic trip device having a movable core and a stationary core and, more specifically, to a cam and axle which maintains each moveable core of a multi-core trip device at a consistent gap from an associated stationary core.
2. Description of the Prior Art
Molded case circuit breakers are well known in the art as exemplified by 5,927,484 issued Jul. 27, 1999 to Malingowski et al., U.S. Pat. No. 5,831,501 issued Nov. 3, 1998 to Kolberg et al., and by U.S. Pat. No. 4,503,408 issued Mar. 5,1985 to Mrenna et. al., entitled xe2x80x9cMolded Case of Circuit Apparatus Having Trip Bar With Flexible Armature Interconnectionxe2x80x9d assigned to the assignee of the present application. The foregoing are incorporated herein by reference.
In molded case circuit breakers in which the power contacts, operating mechanism, and trip unit are mounted inside of a molded plastic insulative housing, a common type of magnetic trip device is a solenoid which includes a stationary core through which the current in the protected circuit is passed. The current passing through the stationary core creates a magnetic field. When there a very high instantaneous currents, such as those associated with a short circuit, the magnetic field intensifies. A plunger assembly, having a moveable core and a plunger tab which engages the trip latch on the operating mechanism, is partially disposed within the stationary core. Typically, a spring provides a limited force biasing the movable core away from the stationary core and preventing the plunger from engaging the trip latch. The force of the spring is overcome by the magnetic field generated by the stationary core during a short circuit. That is, when a short circuit occurs, the current in the stationary core creates a magnetic field strong enough to overcome the moveable core spring thereby allowing the moveable core to move toward the stationary core and causing the plunger to engage the trip latch.
The amount of current required to trip the device can be controlled by adjusting the amount of separation between the plunger assembly and stationary core. When the plunger assembly is located closer to the stationary core, a weaker magnetic field, and therefore a lower current, is required to draw the plunger assembly toward the stationary core to trip the device. In order to adjust the trip condition, the plunger assembly is mounted in a plunger assembly support structure having a base and a moveable plunger carriage. The carriage allows the plunger assembly, including the moveable core, to be moved relative to the stationary core. A carriage is used so that adjusting the gap between the moveable core and the stationary core does not impact on the compression of the moveable core biasing spring. The moveable plunger carriage is coupled to an adjustment device to address the initial gap between the plunger assembly and the stationary core.
As disclosed in Malingowski and Kolberg, prior art adjustment devices included adjustment means such as a cam or a rotatable disk having an angled surface. As these adjustment means are rotated, the trip condition is constantly changed, not unlike an analog device. That is, for every point for which the adjustment means is rotated, the moveable carriage and plunger assembly are moved toward or away from the stationary core, changing the amount of separation between the stationary and moveable cores. This is a disadvantage as users typically want the trip condition set to coincide with a discrete over-current condition. Because the adjustment means of the prior art change the trip condition for each point of rotation, it is difficult to set the trip device to trip at a precise over-current condition. For example, if the adjustment means is coupled to a wheel having a visual indication of various trip conditions and a pointer on the housing of the molded case circuit breaker where the user adjusts the wheel to adjust the trip condition, a user would have to set the wheel to be precisely aligned with, not slightly above or below, the visual indication in order for the trip device to be set at the indicated trip condition.
In a circuit breaker, such as a three phase breaker, having multiple main contacts, and therefore multiple adjustment means, an adjustment device having a smooth transition between various trip conditions is unlikely to place each movable core at the same degree of separation from the associated stationary core. Thus, if each adjustment means of a multiple main contact device is set slightly differently, the breaker will not be set to trip at a precise over-current condition. Additionally, such multiple unit trip devices are typically connected by an extended camshaft. Such a camshaft is subject to flexing which allows each cam to be set at a slightly different angle, and therefore, at a slightly different trip condition. Additionally, variations in the components during manufacture may result in a misalignment between the various trip units.
There is a need, therefore, for a molded case circuit breaker magnetic trip mechanism which sets the trip condition at a precise trip condition regardless of slight variation of the adjustment means.
There is a further need for a molded case circuit breaker magnetic trip mechanism which consistently sets the over-current condition for multiple main contacts within the circuit breaker.
There is a further need for a molded case circuit breaker magnetic trip mechanism which accommodates variations within the manufacturing tolerances of the trip mechanism components.
These needs and others are satisfied by the invention which provides magnetic trip adjustment scheme having a plunger assembly carriage coupled to a cam with a plurality of sections, each section having a constant radius. Thus, the plunger assembly carriage is maintained at a specific location so long as it is contacting the cam anywhere on a certain section. Where the circuit breaker has multiple main contacts, alignment of the magnetic trip units are maintained by a camshaft having an elongated coupling and crush ribs.
A molded case circuit breaker includes at least one pair of separable main contacts. The main contacts are disposed in the circuit breaker housing. The circuit breaker may be tripped manually by a handle or by a magnetic trip device. The magnetic trip device includes a rotating trip bar, a plunger assembly which includes a moveable core, and a stationary core. The stationary core is in electrical communication with the load side of the breaker. As electricity flows through the stationary core, a magnetic field is created. When an over-current condition occurs, the magnetic field intensifies, attracting the movable core of the plunger assembly. The plunger assembly includes a tab which contacts the rotating trip bar. When an over-current condition occurs and draws the moveable core towards the stationary core, the plunger tab causes the trip bar to rotate which in turn trips the breaker.
The plunger assembly is mounted in a moveable carriage which is responsive to a cam. By moving the carriage, the plunger assembly can be positioned closer to or further from the stationary core. When the plunger is closer to the stationary core, the magnetic force has a greater attracting effect. Thus, the over-current condition can be changed by moving the position of the plunger assembly relative to the stationary core. The cam, which positions the plunger carriage, is shaped to have a plurality of sections each with a specific constant radius. Each section positions the plunger assembly a specific distance from the stationary core. Each distance is associated with a specific and discrete over-current condition. The cam is coupled to a control mechanism, such as a wheel having a visual indication of the trip condition. Because the cam sections have a constant radius, a user does not have to set the control mechanism precisely. So long as the contact point between the moveable carriage and the cam is on the appropriate section of the cam, the carriage will be set to the indicated trip condition.
Additionally, when a circuit breaker has multiple main contacts, and therefore, multiple trip mechanisms, alignment between the trip mechanisms is more easily achieved by virtue of the cams with constant radius sections. This is because, even if the cams were at slightly different angles, the constant radius sections will maintain each carriage at the same distance from the stationary core. Thus, each unit will be set to trip at the same over-current condition. Additionally, when a trip mechanism has multiple units connected by an extended camshaft, this invention provides a camshaft which resists flexing so that each cam is angularly aligned.